The objective of this project is to contribute to a more detailed understanding of the electronic and vibrational structure of the class of porphyrin molecules and their chemical interactions with external ligands by application of the methods of optical spectroscopy and electron spin resonance. Sharp line spectra are obtained by the use of low temperatures and crystalline host matrices for the guest porphyrin molecules. Selective excitation spectroscopy using a tunable dye laser will be used to separate the spectra originating from inequivalent site species. The interactions of porphyrins with chemical ligands will be studied by observing shifts in the positions of sharp spectral components of porphyrins spectra which can be correlated with the presence of specific chemical ligands.